Electrorefining is a well-known industrial process for producing metals with a very high purity level. For example, in a copper electrorefining process, unrefined impure copper anodes are hung vertically between pure metal cathode starter sheets in an electrolytic cell filled with an aqueous solution of copper sulphate and sulphuric acid, which is known as the electrolyte. An electrical current is passed through the electrolytic cell and copper gradually dissolves from the anode into the electrolyte and then plates onto the cathode. Provided that they meet specific purity and morphology standards, the copper cathodes are shipped and sold to various manufacturers of copper products.
For economic reasons, the electrolyte is recycled and reused as many times as possible. However, one of the known problems is that the electrolyte becomes gradually contaminated with impurities contained in the anode. Antimony is one such impurity which is particularly harmful to the electrolytic process. In copper electrorefining processes, antimony tends to co-deposit on the copper cathodes, thus reducing their purity and commercial value. Therefore, industrial copper electrorefineries must keep the antimony concentrations below certain limits to prevent antimony from contaminating the copper cathodes.
U.S. Pat. Nos. 4,559,216 and 5,366,715 describe methods for removing antimony from copper electrolytes in an electrorefining process. These methods involve contacting the electrolyte with an ion exchange resin having aminophosphonic groups (—NH—CH2—PO3H2), such as Duolite™ C-467 manufactured by Rohm and Haas (USA), and UR-3300 and MX-2 manufactured by Unitika (Japan), to remove the antimony present in the electrolyte.
After the antimony ions have been adsorbed on the resin and the resin has been separated from the electrolyte and washed, it becomes necessary to desorb the antimony from the resin (a procedure known in industry as “elution”) so that the resin may be reused and the antimony may be recovered or disposed of.
Current industrial practice is to desorb or elute the antimony ions from the resin by using a concentrated hydrochloric acid (HCl) solution, which can be subsequently recovered by distillation. After the ion exchange resin has been eluted, the resin can be reused to adsorb more antimony ions from the copper electrolyte and the recovered hydrochloric acid can be reused to elute further antimony ions from the resin.
The main drawback of this ion exchange method is that the resin becomes inactive or “poisoned” after a number of repetitions of the adsorption and elution steps. As shown in the article “A Study of the Ion Exchange Removal of Antimony(III) and Antimony(V) from Copper Electrodes” by P. A. Riveros, J. E. Dutrizac and R. Lastra, published in the Canadian Metallurgical Quarterly, Volume 47, Number 3, pages 307-315, 2008, the poisoning of the ion exchange resin is caused by the gradual accumulation of antimony(V) in the resin phase. This accumulation occurs because the elution rate of antimony(V) with hydrochloric acid is much slower than the elution rate of antimony(III). As a result, the eluted resin that is recycled to adsorption usually contains small amounts of antimony(V). After each adsorption/elution cycle, the concentration of antimony(V) in the resin phase increases gradually, causing a decrease in the resin loading capacity and, eventually, the formation of antimony-bearing compounds in the pores of the resin and on the resin surface.
Extending the elution time until all the antimony(V) has been desorbed is impractical and costly because it would tie up a significant amount of the resin in the elution step, leading to an increase in resin inventory, the plant size and the associated capital and operating costs. In addition, the volume of hydrochloric acid solution required for the elution would increase proportionally, as well as the size and energy consumption of the hydrochloric acid distillation equipment.
Some electrorefineries attempt to reactivate the poisoned ion exchange resin by periodically washing the resin with sodium hydroxide (NaOH). However, this method only causes the resin beads to swell, thereby breaking off any superficial layers of antimony-containing precipitates. Some resin beads break because of the swelling, thereby exposing fresh surfaces on which antimony can be adsorbed. However, sodium hydroxide is not an effective eluting agent for either antimony(III) or antimony(V) and, therefore, this treatment only causes a short-lived reactivation. A further drawback to this proposed approach is that subjecting the ion exchange resin to repeated contacts with acid and alkaline media significantly weakens the resin's structure and shortens its useful life.
What is therefore needed is a cost effective method for increasing the elution rate of antimony(V) from aminophosphonic resins to fully restore their capacity within a suitable period of time so that the accumulation of antimony(V) in the resin is eliminated and therefore the need for periodic replacement of the resin can be significantly reduced or avoided entirely.